A rapid cooling around 6200 BC was first identified by Swiss botanist
Heinrich Zoller [
de] in 1960, who named the event the Misox oscillation (for the
Val Mesolcina).[4] It is also known as the Finse event in
Norway.[5] Evidence for the 8.2 ka event has been found in speleothem records across Eurasia, the Mediterranean, South America, and southern Africa and indicates the event was globally synchronous.[6] The strongest evidence for the event comes from the
North Atlantic region; the disruption in climate shows clearly in
Greenlandice cores and in
sedimentary and other records of the temperate and the tropical North Atlantic.[7][8][9] It is less evident in ice cores from
Antarctica and in South American indices.[10][11] The effects of the sudden temperature decrease were global, however, most notably in changes in
sea level.
Cooling event
The event may have been caused by a large meltwater pulse,[12] which probably resulted from the final collapse of the
Laurentide Ice Sheet of northeastern North America,[13][14][15] most likely when the
glacial lakesOjibway and
Agassiz suddenly drained into the North Atlantic Ocean.[16] The same type of action produced the
Missoula floods that formed the
Channeled Scablands of the
Columbia River basin. The meltwater pulse may have
affected the North Atlantic
thermohaline circulation,[17][18][19] reducing northward heat transport in the Atlantic and causing significant North Atlantic cooling.[20] The
Atlantic meridional overturning circulation (AMOC) weakened by 55%[14] or 62%.[20] Estimates of the cooling vary and depend somewhat on the interpretation of the proxy data, but decreases of around 1 to 5 °C (1.8 to 9.0 °F) have been reported. In Greenland, the event started at 8175 BP, and the cooling was 3.3 °C (decadal average) in less than 20 years. The coldest period lasted for about 60 years, and its total duration was about 150 years.[2] The meltwater causation hypothesis is, however, considered to be speculation[by whom?] because of inconsistencies with its onset and an unknown region of impact.[citation needed]
Researchers suggest that the discharge was probably superimposed upon a longer episode of cooler climate lasting up to 600 years, and it was merely one contributing factor to the event as a whole.[21]
Further afield from the Laurentide Ice Sheet, some tropical records report a 3 °C (5.4 °F) cooling, based on cores drilled into an ancient
coral reef in
Indonesia.[22] The event also caused a global CO2 decline of about 25 ppm over about 300 years.[23] However, dating and interpretation of other tropical sites are more ambiguous than the North Atlantic sites. In addition, climate modeling shows that the amount of meltwater and the pathway of meltwater are both important in perturbing the North Atlantic thermohaline circulation.[24]
The initial meltwater pulse caused between 0.5 and 4 m (1 ft 8 in and 13 ft 1 in) of
sea-level rise. Based on estimates of lake volume and decaying ice cap size, values of 0.4–1.2 m (1 ft 4 in – 3 ft 11 in) circulate. Based on sea-level data from the Mississippi Delta, the end of the Lake Agassiz–Ojibway (LAO) drainage occurred at 8.31 to 8.18 ka and ranges from 0.8 to 2.2 m.[25] The sea-level data from the Rhine–Meuse Delta indicate a 2–4 m (6 ft 7 in – 13 ft 1 in) of near-instantaneous rise at 8.54 to 8.2 ka, in addition to 'normal' post-glacial sea-level rise.[26] Meltwater pulse sea-level rise was experienced fully at great distance from the release area. Gravity and rebound effects associated with the shifting of water masses meant that the sea-level fingerprint[colloquialism] was smaller in areas closer to the
Hudson Bay. The
Mississippi Delta records around 20%, Northwestern Europe 70% and Asia records 105% of the globally averaged amount.[27] The cooling of the 8.2-kiloyear event was a temporary feature, but the sea-level rise of the meltwater pulse was permanent.
In 2003, the
Office of Net Assessment (ONA) at the
United States Department of Defense was commissioned to produce a study on the likely and potential effects of a modern climate change.[28] The study, conducted under ONA head
Andrew Marshall, modeled its prospective climate change on the 8.2 ka event, precisely because it was the middle alternative between the Younger Dryas and the milder Little Ice Age.[29]
Effects
Across much of the world, the 8.2 ka event engendered drier environmental conditions.[30] Northern Hemisphere monsoon precipitation declined by 12.4% for every °C of global mean temperature change, while Southern Hemisphere monsoon precipitation rose by 4.2%/°C.[31] The 8.2 ka event was also associated with an increase in ocean salinity and terrestrial dust flux.[32]
North Africa and Mesopotamia
Drier conditions were notable in
North Africa; the area around the Charef River in eastern Morocco records an episode of extreme aridity around 8,200 BP.[33]East Africa was significantly affected by five centuries of general
drought. In
West Asia, especially
Mesopotamia, the 8.2-kiloyear event was a 300-year
aridification and cooling episode, which may have provided the natural force for Mesopotamian irrigation agriculture and surplus production, which were essential for the earliest formation of classes and urban life.[citation needed] However, changes taking place over centuries around the period are difficult to link specifically to the approximately 100-year abrupt event, as recorded most clearly in the Greenland ice cores.
In particular, in
Tell Sabi Abyad, Syria, significant cultural changes are observed at c. 6200 BC; the settlement was not abandoned at the time.[34]
Madagascar
In northwestern
Madagascar, the 8.2 ka event is associated with a negative δ18O excursion and
calcite deposition, indicating wet, humid conditions caused by the southward migration of the ITCZ.[35] Summer monsoons in the Southern Hemisphere likely became stronger, contributing to precipitation increases.[36] Humidification was two-phased, with an 8.3 kiloyear sub-event preceding the 8.2 kiloyear sub-event by about 20 years.[37]
Europe
The sediment core records of the
Fram Strait show a short-lived cooling during the 8.2 ka event superimposed on a broader interval of warm climate.[38] In western
Scotland, the 8.2 ka event coincided with a dramatic reduction in the Mesolithic population.[39] In the Iberian Peninsula, the 8.2 ka event is linked to greater summer aridity that caused an increase in the frequency of fires and a consequent expansion of fire-resistant evergreen oak trees.[40]
North Asia
Lacustrine sediment records show that Western Siberia underwent humidification during the 8.2 ka event.[41]
South Asia
Carbonates from Riwasa Palaeolake show a weakening of the Indian Summer Monsoon (ISM) synchronous with the 8.2 ka event.[42] Stalagmites from Kotumsar Cave[43] and from Socotra and Oman further confirm the ISM precipitously diminished in strength.[44]
East Asia
A sediment core from
Lop Nur in the
Tarim Basin shows a major dry spell occurred during the 8.2 ka event.[45] The impact of the 8.2 ka event on
forests in the
Korean Peninsula was severe, shown by a sizeable reduction in
pollen production. It took approximately 400 years for forest
ecosystems to recover from the event to their state before the climatic perturbation.[46]
Southeast Asia
Evidence from the
Gulf of Thailand reveals that a sea level drop occurred concordantly with the 8.2 ka event. Also detectable from
palynological and
sedimentological records is an increase in runoff.[47]
North America
In Greenland, the 8.2 ka event is associated with a large negative spike in ice core δ18O values.[48][49] The waters off
Cape Hatteras experienced a major
salinity increase.[50]Batguano δ13C and δD values in the
Grand Canyon declined.[51] Southwestern Mexico became significantly drier, evidenced by the interruption of stalagmite growth.[52] In the
Gulf of Mexico, bay-head deltas back stepped as sea levels rose.[53]Mustang Island was breached and ceased to be an effective salinity barrier.[54] Gulf of Mexico δ18Oseawater values dropped by 0.8%.[55]
South America
The South American Summer Monsoon (SASM) drastically intensified during the 8.2 ka event as revealed by sediment records from Juréia Paleolagoon.[56]
^Zoller, Heinrich (1960). "Pollenanalytische Untersuchungen zur Vegetationsgeschichte der insubrischen Schweiz". Denkschriften der Schweizerischen Naturforschenden Gesellschaft (in German). 83: 45–156.
ISSN0366-970X.
^Ehlers, Jürgen; Gibbard, Philip L. (2004). Quaternary Glaciations – Extent and Chronology. Part II: North America. Amsterdam, The Netherlands: Elsevier. pp. 257–262.
ISBN978-0-444-51592-6.